Travelers shall be very careful by choosing where to buy the Deutschland Ticket.
Customers of D-Ticket.com or D-Ticket.su failed with their tickets' validity when checked and needed to pay 60 EUR fine. Anyone who has bought their ticket there should check its validity as soon as possible and possibly buy a new one. See full article of tech magazine Heise in German language.
Do not buy from platforms which offer that you can book also parts of a calendar month which is by terms and conditions for this kind of ticket not possible. [Edit for clarity: only in first month you can use and pay the ticket pro rata].
Best is to buy directly from Deutsche Bahn or from one of the local transport providers from large German cities or communities. Examples:
BVG: https://www.bvg.de/en/subscriptions-and-tickets/subscriptions/deutschland-ticket
HVV: https://www.hvv-deutschlandticket.de/en/
MVV: https://www.mvv-muenchen.de/en/tickets-and-fares/frequent-travellers/deutschland-ticket-d-ticket/index.html
RMV: https://www.rmv.de/c/en/fahrkarten/sortiment-verkauf/fahrkarten-im-ueberblick/fahrkarten-im-abo/deutschlandticket